Electric power assisted steering systems that use the principle of superimposing a torque and electric power assisted steering systems that use the principle of superimposing a steering angle are generally known. Such systems make use of a reduction gear, such as a worm drive or worm gear comprising a worm gear shaft and a worm gear wheel. The worm gear wheel is generally made of a synthetic material that may change size in the presence of varying environmental conditions, such as, for example, temperature and moisture. Moreover, the worm gear wheel may change its size over the course of its serviceable life as a result of wear.
Manufacturing tolerances in the components of the worm drive also influence the amount of play in the meshing arrangement between the worm gear wheel and the worm gear shaft. Such play in this meshing arrangement can be problematic owing to the fact that, for example, noises can occur in the meshing arrangement as the direction of the loading changes. Furthermore, the magnitude of the transmitted power can be reduced and the driver may notice a change in the steering sensation.
It may, therefore, be advantageous to provide an electric power assisted steering system in which play in the meshing arrangement between the worm gear shaft and the worm gear wheel is substantially reduced or eliminated. Moreover, the system is simplified and utilizes minimal components to eliminate play in the meshing arrangement in order to reduce costs, facilitate the assembly process, and to keep the required installation space as small as possible.